Fixed-wing aircraft and helicopter control surfaces such as flaps, rudders and helicopter blades are typically positioned for the control of the aircraft by hydraulic actuators. Such hydraulic actuators usually include a piston connected to the control surface by a connecting rod and reciprocable within a cylinder, selective pressurization of the cylinder on opposite sides of the piston positioning the piston and thus the control surface connected thereto. It is common for a single control surface to be positioned by pairs of actuators whereby, in the event that one of the actuators is damaged, a redundant actuator may set the control surface for continued operation of the aircraft. Thus, it will be understood that if damage to one of the actuators is manifested in a jamming of the piston within the cylinder, the other actuator must not only power the control surface in the normal movement thereof, but must also overcome such jamming by movement of the piston of the damaged actuator past the point of the jam. Where, in the case of military aircraft, the jammed actuator has been damaged ballistically, such jamming is often the result of a rupture of the cylinder from the exterior thereof by a projectile such as a bullet. Such a rupture will inwardly deform a portion of the cylinder wall forming an obstruction therein to traversal of that portion of the cylinder by the piston. When the projectile strikes the connecting rod, damage to the rod may include cratering of the rod or formation of protuberances thereon which jam in the actuator cylinder gland (end seal portion).
In an effort to devise piston structures capable of traversing obstructions in the cylinder walls, and cylinder glands capable of allowing damaged connecting rods to pass therethrough, it has been proposed that the pistons and glands be provided with frangible portions whereby upon encountering an obstruction in the cylinder wall, or connecting rod, the affected frangible portion will rupture around the obstructions to allow the piston to move past the obstruction or the damaged rod to pass through the gland. In one such actuator, the piston and gland are provided with a plurality of circumferentially spaced segments defining a groove which receives an annular seal, the segments being spaced by radially extending slots and being frangible along a weakened portion or scribe line. The frangible segments may also be provided with an annular rib which upon encountering an obstruction, provides a mechanical advantage in breaking off the segments.
It will be appreciated by those skilled in the art that the provision of such a scribe line, ribs and slots in such actuators necessarily contributes to the complexity and cost of the actuator. Additionally, the provision of the radially directed slots adds to the risk of actuator fluid leakage past the piston under conditions of normal operation. It will be apparent that any piston or gland structures having a frangible portion, necessarily represents a compromise in fatigue strength required in normal operation. Similarly, pistons and glands of a more simple (uniform) shape but composed of a brittle material which breaks when encountering ballistic damage also represent sacrifices in fatigue strength required in normal actuator operation.
Composite actuator cylinders which absorb impact energy for purposes of preventing damage to the interior of the cylinder have been proposed. However, it would seen that such cylinders may be quite heavy and bulky for their intended use and that complete assurance of the prevention of damage to the interior of the cylinder, difficult at best to attain.